The alignment of a gas laser cavity having one or two external Brewster windows can be accomplished by using a laser beam directed through the cavity. The external mirrors are moved into alignment to create a lasing action in the laser cavity and then the mirrors are permanently fixed when the output of the laser being aligned is optimized. However, many laser devices use mirrors which are rigidly attached to the laser bore tube. It is difficult with this type of laser device to tune the cavity by aligning the mirrors without operating the laser cavity being aligned. This requires that the laser bore tube be evacuated and then filled with the laser gas and connected into a circuit to provide an operating laser device. A definite advantage providing an economy in fabrication results if the laser bore tube can be separately aligned with the mirrors without it forming a part of an operating laser.
A further complication of aligning the mirrors and laser bore of the optical cavity is that the mirrors normally are formed as reflective coatings applied to surfaces of small blocks of transparent material, such as glass. If the alignment of the laser device is done without actually operating the laser with a gaseous medium, it is then necessary to direct aligning light beams through the mirror blocks and substantially normal to the mirror coatings on the block surfaces. The mirror blocks are not accurately formed and the opposite surfaces are usually not parallel to the mirror coatings and thus are not normal to the bore axis of the optical cavity. In fact, it is recognized that the mirror blocks are effectively small wedges of transparent material which preclude a straight line passage of an aligning light beam through the block without refraction.